1. (Field of the Invention)
The present invention generally relates to an automobile underbody structure and, more particularly, to an underbody structure designed to substantially withstand a lateral impact applied to the automobile in a direction widthwise thereof.
2. (Description of the Prior Art)
It is quite well known that a floor panel forming the floor inside an automobile has a tunnel defined therein so as to protrude inwardly of the automobile compartment while extending longitudinally of the automobile for accommodating therebelow a rear end portion of a transmission and a drive shaft drivingly coupled with the transmission for the transmission of an engine drive to rear driven wheels. On opposite side edges of the floor panel are rigidly mounted side sills of generally polygonal cross-sectional shape extending longitudinally of the automobile, which side sills are positioned immediately below side access openings adapted to be selectively closed and opened by respective side door assemblies.
It is also well known that the floor panel is reinforced by longitudinal side frames each positioned between the tunnel and one of the side sills. Each of these longitudinal side frames has a generally U-shaped cross-section having a pair of flanges protruding laterally outwardly from the opposite side edges of the side frame, through which flanges the side frame is welded to the undersurface of the floor panel so as to extend in a direction longitudinally of the automobile.
Hitherto, numerous attempts have been made to increase the rigidity of the automobile underbody structure against any possible impact which would be applied thereto in the event of, for example, a side collision, i.e., the collision of one automobile against a different automobile in a direction laterally thereof, and/or to minimize the transmission of shocks or vibrations from the wheels to the automobile body which would occur in the event that the automobile is used in an off-road environment. One such attempt includes the use of so-called torque boxes secured from below to the floor panel at a front portion thereof, one floor torque box being positioned on each side of the tunnel in the floor panel.
The torque box referred to above is a reinforcement member of generally U-shaped cross-section having a pair of flanges protruding laterally outwardly from the opposite side edges thereof and secured rigidly through the flanges to the undersurface of a front end portion of the floor panel so as to connect the associated side frame and the associated side sill together while defining a generally closed cross-section between it and the floor panel.
Japanese Laid-open Utility Model Publication No. 58-46668, published in 1983, discloses one approach to increase the rigidity of the automobile underbody structure against lateral impact for the purpose of minimizing damage which passengers may suffer in the even of a side collision. According to this publication, the floor panel has rear risers for the connection thereto of legs of a front seat assembly, of which are secured to opposite lateral walls of the tunnel and the remaining two of which are secured in part to the floor panel and in part to the respective side sills. The rear risers adjacent the opposite side edges of the floor panel are positioned generally below center pillars on respective sides of the automobile body structure, each such center pillar separating a respective front side access opening from an adjacent rear side access opening.
The automobile underbody structure disclosed in this publication makes use of a sub-cross member secured from below to the undersurface of the floor panel and connected at one end to a portion of each side edge of the floor panel immediately beneath the associated rear riser and at the opposite end to the adjacent longitudinal side frame.
This prior art approach such as exemplified by the above discussed Japanese publication has been found inapplicable to an automobile of a convertible type, i.e., a model not having a rigid roof. More specifically, in the convertible model, since the automobile body structure does not include a rigid roof, the rigidity of the automobile body structure as a whole is low compared with a normal automobile body structure having a rigid roof. Therefore, the automobile body structure for a convertible model is relatively apt to bend or twist under the influence of vibrations induced in the automobile body structure during operation thereof and/or lateral forces such as occur during cornering. Accordingly, with the prior art approach, there often are found problems associated with the lack of sufficient rigidity of the automobile front underbody structure.
Also, it is well-known that side door assemblies are generally hingedly connected to respective hinge pillars, each extending upwardly from a front end of the associated side sill, for selectively closing and opening the associated side access openings. Considering this very usual practice to connect the side door assemblies to the hinge pillars, the convertible model requires that each hinge pillar have an increased rigidity.
Furthermore, in the convertible model, vibrations transmitted from the automobile power plant, for example, the engine, to the automobile body structure tend to act on the automobile body structure so as to twist the latter and, consequently, resonant vibrations may occur inside the passenger compartment.
The above discussed problems may be substantially eliminated if the floor panel, or at least the front end portion of the floor panel, and members surrounding the floor panel are made of plate material having a substantial thickness. However, an increase of the thickness of the floor panel and the associated members may bring about other problems, for example, increased weight of the automobile body structure, increased manufacturing cost, increased fuel consumption and lowered acceleration performance.